Solid state power factor correction controllers provide power factor correction for switch-mode power converters. Interest in these circuits has surged as a result of a significant increase in the use of switch-mode power converters in such products as computers and electronic lamp ballasts.
The specific problem to which the invention is applicable is the design of a solid-state power factor correction controller that does not require directly sensing the current through the current storage inductor in the switch-mode power converter, while still incorporating pulse-by-pulse current limiting. An advantageous design would be adaptable to multiple converter topologies.
Conventional switch-mode power converters include a bridge rectifier and a large electrolytic capacitor at the input. The converter draws current from the AC line only when the line voltage exceeds the voltage across the capacitor, resulting in sharp current spikes at the peaks of the voltage sinusoid. This current waveform has a very large harmonic content, and harmonic currents perform no useful work.
Power factor for switch-mode converters is defined as the ratio of fundamental in-phase power to total power, a definition that encompasses harmonic currents. Using this definition, a typical converter will have a power factor of 0.7 or less.
This low power factor is undesirable for several reasons. First, the low power factor limits the amount of power which can be extracted from a given circuit. Second, the third harmonic current is summed across the phases of a three-phase circuit, producing a net current flow in the neutral line which in most building distribution circuits is not sized to carry large currents, and thus it may easily be overloaded. Third, although the higher harmonics are largely blocked by power transformers, they may couple through the building distribution circuits, producing EMI (electromagnetic interference) from power lines.
As a result, demand has been increasing in both industry and government circles that switch-mode power converters be power factor corrected. Moreover, while commercial environments are most immediately affected by the power factor problems associated with switch-mode power conversion, extending government requirements for power factor correction to consumer products is expected.
The most practical approach to solid-state power factor correction is to combine a standard switch-mode power conversion topology with a specially designed controller to regulate the switching of the power converter. Basically, these power factor correction (PFC) controllers implement power factor correction by drawing current from the AC line in phase with the line voltage, thereby yielding a reduction in harmonics and EMI. Power factor correction does not replace switch-mode power conversion, but rather, act as a preregulator that takes power from the AC line and provides an unregulated DC source for the switch-mode power converter that follows.
Solid-state PFC controllers have been designed for each of the principal switch-mode power converter topologies: buck, boost, and buck-boost. In general, buck-derived circuits, including buck-boost circuits, interrupt the line input current, while boost-derived circuits do not.
Each of these power converter topologies is described in Section 1.1 of the Detailed Description. Basically, each includes (a) a current storage inductor, and (b) a power switching transistor that controls the current through the inductor and is controlled by the PFC controller to provide power-factor-corrected conversion.
Although buck and buck-boost power converters have their advantages, to date most solid-state PFC controllers have been built around boost-derived power converters. Boost topologies do not interrupt line current, making it easier to block high frequency currents from the AC line (i.e., less robust and expensive filtering is required). Typically, these controllers use current shaping techniques that attempt to ensure that line current is drawn in-phase with the line voltage.
Examples of these PFC controllers are the Micro-Linear ML4812, the Siemens TDA4814A, and the Unitrode UC1854. The Micro-Linear and Siemens controllers are designed for boost-derived topologies, while the Unitrode controller can be used with any topology.
All of these PFC controllers use current-mode control with inner and outer coupled feedback loops. When the power transistor is turned on, inductor current increases linearly. When the current reaches some specific threshold, as sensed by the inner control loop, the power transistor is turned off. The current threshold is itself controlled by a second, outer control loop which senses the load voltage, and therefore, current (i.e., power) demand. If the load voltage is low, the outer control loop raises the current threshold of the inner control loop so as to pump more current into the bulk storage capacitor. Conversely, if the load voltage is high, the outer control loop lowers the current threshold of the inner loop.
Current-mode control can be easily extended to give power factor correction. In a continuous-mode converter, the current threshold of the inner loop directly controls average inductor current, and thus average line current for a boost-mode topology. Thus, all that is necessary is to ensure that the current control signal provided to the inner control loop is proportional to the line voltage. By forcing peak inductor current to follow a sinusoidal envelope, the average line current can be shaped into a sine wave.
A number of controller designs have been used to implement current-mode control. The Micro-linear PFC controller implements continuous mode operations, ensuring that the current control signal provided by the inner control loop is proportional to the line voltage using a multiplier between the inner and outer control loop--one input to the multiplier is a current demand signal calculated by the outer control loop, and the other input is the line rectified waveform. The Siemens PFC controller implements nearly continuous mode operations, using an auxiliary winding on the current storage inductor to indirectly sense inductor current --this sense winding is monitored, and the power transistor is turned on when inductor current falls to zero.
The Unitrode PFC controller is a versatile design that can be used to control both boost- and buck-derived topologies. This controller relies upon directly sensing average inductor current rather than peak inductor current--a small sensing resistor produces a voltage proportional to line current, and this sensing voltage is fed to a current error amplifier where it is compared to a current demand signal generated by an outer control loop (a feedback network around the current error amplifier acts as a low-pass filter, thus ensuring that the demand signal controls average line current rather than peak line current). If the current error amplifier sees that the demand signal is larger than the filtered line current, its output voltage increases, thereby increasing the conduction time of the power transistor.
To provide the current demand signal, the outer control loop implements a line voltage feed forward technique that makes the controller much more insensitive to line voltage fluctuations. The current demand signal is the product of the average load current demand (from the voltage error amplifier), the instantaneous line voltage, and the reciprocal of the square of the average line voltage. This function will appear to be proportional to line voltage at 60 Hz while for lower frequencies, this function will appear to be inversely proportional to line voltage due to the average line voltage term. Thus, the circuit will power factor correct the 60 Hz AC sinusoid, but for lower-frequency variations in line voltage, the circuit will behave as a normal converter and will not be so prone to over-reacting to line voltage fluctuations.
The currently available PFC controller designs for boost topologies are disadvantageous in several respects. A boost circuit must always step up the load voltage, and for many applications this leads to high load voltages (500 V or more), and often to uneconomic solutions and the possibility of safety hazards. Second, boost converters are subject to very high inrush currents when they are started, which can overstress components and are difficult to suppress without complicated circuitry. Third, currently available PFC controllers for boost converters, including the Unitrode controller that can be used with other topologies, require direct sensing of inductor currents using either small-value sensing resistors (which introduce noise) or current sense transformers (which are relatively expensive).
Accordingly, a need exists for a power factor correction controller for switch-mode power converters that is adaptable to multiple converter topologies, and in particular to buck and buck-boost topologies which permit a step-down in voltage, but that does not require sensing the current through the current storage inductor.